Multiple file uploads from browser to S3 via Dropzone.

app.php

<?php

// snippet of app.php setup for setting up the PostPolicy object and the uploader form params

$app['s3_postpolicy'] = $app->share(function($app) {
    $s3policy = new \Aws\S3\PostPolicy(
        $app['aws.key'], $app['aws.secret'],
        $app['aws.s3.upload_bucket'],
        $app['aws.s3.upload_policy_timeframe']
    );

    return $s3policy
        ->addCondition('', 'acl', 'private')
        ->addCondition('', 'bucket', $app['aws.s3.upload_bucket'])
        ->addCondition('starts-with', '$key', '')
        ->addCondition('starts-with', '$Content-Type', '')
        ->addCondition('', 'success_action_status', 200);
});

// Used for building an upload form
$app['uploader'] = $app->share(function($app) {
    return (object)array(
        'bucket' => $app['aws.s3.upload_bucket'],
        'policy' => $app['s3_postpolicy']->getPolicy(true),
        'signature' => $app['s3_postpolicy']->getSignedPolicy(),
        'key' => $app['aws.key'],
        'key_prefix' => hash('sha256', uniqid($app['aws.s3.upload_bucket'], true)),
        'success_status' => 200,
    );
});

PostPolicy.php

<?php

namespace Aws\S3;

/**
 * @category    Amazon
 * @package     AWS
 * @copyright   Copyright 2009 Amazon Technologies, Inc.
 * @link        http://aws.amazon.com
 * @license     http://aws.amazon.com/apache2.0  Apache License, Version 2.0
 * @author      Michael@AWS
 */

/**
 * Create an Amazon S3 POST policy.
 *
 * @package AWS
 */
class PostPolicy {

    /**
     * AWS Secret Access Key.
     *
     * @var string
     */
    protected $_awsSecretAccessKey = '';

    /**
     * AWS Access Key ID.
     *
     * @var string
     */
    protected $_awsAccessKeyId = '';

    /**
     * Amazon S3 bucket.
     *
     * @var string
     */
    protected $_bucket = '';

    /**
     * Array of conditions.
     *
     * @var array
     */
    protected $_conditions = array();

    /**
     * Duration in seconds that the policy is valid.  Default is 24 hours.
     *
     * @var int
     */
    protected $_duration = 86400;

    /**
     * Maintain a cache of un-encoded policy so that multiple requests to get
     * the policy will not incur unnecessary computation.
     *
     * @var string
     */
    protected $_policyCache = '';

    /**
     * Construct a new POST policy object.
     *
     * @param string $awsSecretAccessKey
     *      Your AWS Secret Access Key.
     *
     * @param string $bucket
     *      Amazon S3 bucket name.
     *
     * @param int $duration
     *      The number of seconds for which the policy is valid.
     */
    public function __construct($awsAccessKeyId, $awsSecretAccessKey, $bucket = '', $duration = 86400) {
        $this->_awsAccessKeyId = $awsAccessKeyId;
        $this->_awsSecretAccessKey = $awsSecretAccessKey;
        $this->_bucket = $bucket;
        $this->_duration = $duration;
        $this->_expireCache();
    }

    /**
     * Add a policy condition.
     *
     * @param string $condition
     *      Condition type.  Possible values are 'eq', 'starts-with',
     *      and 'content-length-range'.  Pass null or an empty string to set a
     *      condition that uses the {"field": "match"} syntax.  Passing anything
     *      in the $condition parameter will cause the condition to use the
     *      [condition, field, match] syntax.
     *
     * @param string $field
     *      The field name.
     *
     * @param string $match
     *      String to match.
     *
     * @return Aws_S3_PostPolicy
     *      Returns a reference to the object.
     *
     * <code>
     * $policy->addCondition('', 'acl', 'public-read');
     * // Produces: {"acl": "public-read"}
     *
     * $policy->addCondition('eq', 'acl', 'public-read');
     * // Produces: ["eq", "$acl", "public-read"]
     *
     * $policy->addCondition('starts-with', '$key', 'user/betty/');
     * // Produces: ["starts-with", "$key", "user/betty/"]
     *
     * $policy->addCondition('content-length-range', 1048579, 10485760);
     * // Produces: ["content-length-range", 1048579, 10485760]
     * </code>
     */
    public function addCondition($condition = 'eq', $field, $match = '') {
        $this->_conditions[] = array($condition, $field, $match);
        $this->_expireCache();
        return $this;
    }

    /**
     * Get the AWS Access Key ID associated with this POST policy.
     *
     * @return string
     *      Returns the AWS Access Key ID.
     *
     */
    public function getAwsAccessKeyId() {
        return $this->_awsAccessKeyId;
    }

    /**
     * Get the bucket associated with the policy.
     *
     * @return string
     *      Returns the Amazon S3 bucket name.
     */
    public function getBucket() {
        return $this->_bucket;
    }

    /**
     * Get a spefic permission based on the field name.
     *
     * @param string $field
     *      Field name to retrieve.
     *
     * @param bool $fullCondition
     *      Whether or not to return the entire condition as an array or just
     *      retrieve the match value of the condition.  By default, this is set
     *      to false so that it will only retrieve the match value of the
     *      condition.
     *
     * @return array|string
     *      If $fullCondition is set to true, then this method will return an
     *      indexed array of data about the condition:
     *      array(condition, field, match).
     *      Returns an empty array if the field is not found.
     *
     *      If $fullCondition is set to false, then this method will only return the
     *      match field of the condition.
     */
    public function getCondition($field, $fullCondition = false) {
        if (count($this->_conditions)) {
            foreach ($this->_conditions as $condition) {
                if ($condition[1] === $field) {
                    return ($fullCondition) ? $condition : $condition[2];
                }
            }
        }
        return ($fullCondition) ? array() : null;
    }

    /**
     * Get all of the conditions associated with the policy.
     *
     * @return array
     *      Returns an indexed array of conditions.
     */
    public function getConditions() {
        return $this->_conditions;
    }

    /**
     * Retrieve the POST policy.
     *
     * @param bool $encode
     *      Set to true to retrieve the encoded policy.
     *
     * @return string
     *      Returns an un-signed POST policy.
     */
    public function getPolicy($encode = false) {
        if (!$this->_policyCache) {
            $policy = sprintf('{ "expiration": "%s"', gmdate('Y-n-d\TH:i:s.000\Z', time() + $this->_duration));
            if (count($this->_conditions)) {
                $policy .= ', "conditions": [';
                $first = true;
                foreach ($this->_conditions as $condition) {
                    if (!$first) {
                        $policy .= ', ';
                    }
                    if ($condition[0]) {
                        $policy .= sprintf('["%s", "%s", "%s"]', $condition[0], $condition[1], $condition[2]);
                    } else {
                        $policy .= sprintf('{"%s": "%s"}', $condition[1], $condition[2]);
                    }
                    $first = false;
                }
                $policy .= ']';
            }
            $policy .= '}';
            $this->_policyCache = $policy;
        } else {
            $policy = $this->_policyCache;
        }
        if (!$encode) {
            return $policy;
        } else {
            return base64_encode(utf8_encode(preg_replace('/\s\s+|\\f|\\n|\\r|\\t|\\v/', '', $policy)));
        }
    }

    /**
     * Get a signed POST policy.
     *
     * @return string
     *      Returns a signed POST policy.
     */
    public function getSignedPolicy() {
        return base64_encode(hash_hmac('sha1', $this->getPolicy(true), $this->_awsSecretAccessKey, true));
    }

    /**
     * Expire the policy cache.
     *
     * @return Aws_S3_Policy
     *      Returns a reference to the object.
     */
    protected function _expireCache() {
        $this->_policyCache = '';
        return $this;
    }

    /**
     * Reset the policy by clearing the conditions, removing the bucket name,
     * and making the duration 86400 seconds (24 hours).
     *
     * @return Aws_S3_PostPolicy
     *      Returns a reference to the object.
     */
    public function reset() {
        $this->_conditions = array();
        $this->_bucket = '';
        $this->_duration = 0;
        $this->_expireCache();
        return $this;
    }

    /**
     * Set the Amazon S3 bucket.
     *
     * @param string $bucket
     *      Amazon S3 bucket name.
     *
     * @return Aws_S3_PostPolicy
     *      Returns a reference to the object.
     */
    public function setBucket($bucket) {
        $this->_bucket = $bucket;
        return $this;
    }

    /**
     * Set the number of seconds the policy is valid.
     *
     * @param int $seconds
     *      The number of seconds the policy should be valid.
     *
     * @return Aws_S3_PostPolicy
     *      Returns a reference to the object.
     */
    public function setDuration($seconds) {
        $this->_duration = $seconds;
        $this->_expireCache();
        return $this;
    }
}

upload_form.html.twig

<form action="https://{{ app.uploader.bucket }}.s3.amazonaws.com/" id="s3dropzone">
  <input name="key" type="hidden" value="{{ app.uploader.key_prefix }}/${filename}">
  <input name="acl" type="hidden" value="private">
  <input name="policy" type="hidden" value="{{ app.uploader.policy }}">
  <input name="signature" type="hidden" value="{{ app.uploader.signature }}">
  <input name="content-type" type="hidden" value="application/octet-stream">
  <input name="AWSAccessKeyId" type="hidden" value="{{ app.uploader.key }}">
  <input name="success_action_status" type="hidden" value="{{ app.uploader.success_status }}">
</form>

I have 2 questions

<input name="success_action_status" type="hidden" value="{{ app.uploader.success_status }}">

According to:

http://aws.amazon.com/articles/1434

success_action_status is not a validate field, is it success_action_redirect?

When I set success_action_redirect field in the form, dropzone js will end up with an error says "Sever responded with code 0"

how do you handle the response from aws?